We propose to continue our investigations of lung growth and lung maturation: (a) Lung maturation: We will continue evaluation of fibroblast-pneumonocyte factor, an oligopeptide produced by the fetal lung fibroblast in response to glucocorticoids which stimulates differentiation of the alveolar Type II cell in vitro and accelerates lung maturation in vivo. This substance will be further purified and chemically characterized followed by development of radioreceptor and/or radioimmunoassays. Regulation of its production and effects, both in vitro and in vivo, will be studied. Further attempts to identify and quantify this material in various biological fluids (e.g. amniotic fluid) will be made. (b) Lung growth: We have recently identified somatomedin-like activity in sera collected from pneumonectomized rabbits which stimulates thymidine incorporation into DNA by alveolar Type II cells in vitro (but not lung or skin fibroblasts). Further evaluation will include specific measurement of Somatomedin C (SMC) in these sera, determination of the time course of appearance of biologic activity and of SMC following penumonectomy, and the effect of growth hormone and hypophysectomy on these parameters. We believe that such a combined in vitro/in vivo approach should provide a powerful tool with which to probe the regulation of lung growth and lung maturation.